In the manufacture of MOS-type semiconductor structures, particularly those having small geometries associated with integrated circuits (IC's), the problem of making electrical contact to the source, drain, and gate of the device has been a difficult one. In the prior art, it is known to form an oxide layer over the source, drain, and gate regions to which connection is to be made, followed by masking and etching away of the oxide in the regions in which contact is to be made. Metal is then deposited onto the underlying source, drain, and gate regions exposed by the etching to serve as an electrical interconnect to those regions. In the prior art, it is conventional to accomplish the etching step by wet chemical etching or plasma etching. However, it has been found that the wet chemical etching resulted in severe undercutting, producing oversized contact windows, which make it difficult to achieve minimum contact size. Additionally, the undercutting increases the probability of source-drain shorting to the gate electrode. Plasma etching, too, exhibits a number of difficulties including a slow etching rate for SiO.sub.2, an insufficient etch rate difference between SiO.sub.2 and underlying silicon, and the production of extremely steep walls in the etched oxide. Subsequent metal overlays display serious metal step-coverage problems.
Another approach is discussed by V. L. Rideout in a paper entitled, "Polysilicon-Gate Field-Effect Transistors with Self-Registering Metal Contacts to both Polysilicon and Diffused Silicon Regions", IBM Technical Disclosure Bulletin, Volume 21, No. 9, Feb. 1979. In this reference there is taught a method for fabricating field-effect transistors in which several nitride and oxide layers are utilized in the processing to achieve self-registering metal contacts to both the diffused regions in the substrate and to polysilicon gates at the surface of the device.